1. Field of the Invention
This invention relates in general to method and apparatus for opto-electronically scanning masters which are mounted on a transparent master carrier and can be used for scanning devices for flat bed or drum scanning apparatus.
2. Description of the Prior Art
Flat bed or drum scanning devices are employed in facsimile transmission technology wherein a light beam scans a master to be reproduced by scanning it image point-wise and line-wise. The scanning light from the master impinges on an opto-electronic scanning element and is converted into an image signal. The image signal is sent over a transmission channel to a facsimile recording means in which the reproduction of the master is recorded.
Assembled masters called paste-ups comprise a number of individual masters for lettering and images which are assembled into a full page on a master support. The individual masters can either be opaque masters or transparency masters and the paste-ups are correspondingly scanned by reflection for opaque copy or by transmission for transparencies.
When scanning occurs of montage edges of the individual masters which are referred to as shadow edges and if scratches occur on the masters, the scanning light is diffusedly reflected or respectively transmitted with an asymmetrical intensity distribution which leads to an incorrect measurement given nearly uniform light measurement by the scanning element and, thus, results in an amplitude error in the image signal. Due to such amplitude errors, montage or shadow edges are reproduced when recording the paste-ups as disruptive black edges.
It is known in the prior art to illuminate paste-ups either with two light sources which are disposed to the left and right of the scanning line or to pickup the scanning light with two opto-electrical transducers mounted to the left and right of the scanning line. The amplitude errors can be reduced by this technique but cannot be eliminated.
It is also known to employ scanning elements having light pickup devices which collect the asymmetrical scanning light under a solid angle which is as large as possible and supply it to an opto-electronic transducer. However, the amplitude errors can only be reduced and cannot be entirely eliminated with such light pickup devices.